It's possible that it suffered paint loss at some 
point, and a collector masked this by painting 
over the front of the cel.  

It's unusual (in my limited experience) to find 
cels with extensive painting on the front.  I do 
have three cels that were repainted on the front, 
in two cases to correct errors committed by the 
cel painters.  (Maybe in the 3rd case too, but 
since I don't have the douga for it I can't 
tell.)  So it's possible that the studio had the 
cel corrected in this way.  

But Mandarake might have had the same suspicion 
that I had--that it's been "retouched" by a 
previous owner to conceal damage.

(This, of course, doesn't mean that the suspicion 
is correct ... in which case you're in luck!)